Question

• When soluble phosphate fertilizer (as monocalcium phosphate monohydrate, Ca(H2PO4)2•H2O) is applied to calcareous soil, which phosphate mineral that initially form?

• There are some calcium phosphate minerls (e.g., brushite [CaHPO4•2H2O], m onetite [CaHPO4], octacalcium phosphate [Ca8H2(PO4)6•5H2O], apatite [Ca5 (PO4)3(OH,F)]). Using the concept of "STEP RULE", list the sequence of the p hosphate mineral formation in METASTABILITY state.

Answer 1

A convenient and economical process for making a fertilizer containing soluble phosphate is to react phosphoric acid with the raw ore to form triple super phosphate (TSP). In this beneficiation process, the total P that becomes available is the sum of the phosphate added as acid plus the phosphate released from the ore by the acid attack. The reaction of interest may be written as in reaction 1:

14H3 PO4+ Ca10 (PO4 )6 (OH)2→10[Ca(H2 PO4 )2. H O)] 2H2O (1)

                This idealized reaction implies that phosphoric acid reacts preferentially with the hydroxyapatite (HA) (or fluoroapatite) in the ore and not with the main impurities (iron oxides, alumina, and silicates). To improve the contents in soluble phosphorus, i.e., the purity of the TSP, it is possible to dissolve the reaction products in water and remove the insoluble fraction by filtration, thus recovering the soluble monocalcium phosphate monohydrate (MCPM) in solution. However, it is known from the thermodynamics of the Ca(OH)2− H3PO4−H2O system that HA is the most stable product.4 In other words, any solid with Ca/P < 1.67 will hydrolyze in solution to release phosphoric acid and form HA or other reaction products with higher Ca/P ratio than the starting solid. It is expected, for instance, that MCPM will hydrolyze according to reaction 2:

[Ca(H2PO4)2H2O)] → CaHPO4+ H3PO4 (2)

On the other hand, the reactions do not go to completion, unless a mechanism is present to remove the acid. Equilibrium will be reached at a pH value where the composition of the solution reaches the intersection of the solubility isotherms, when the solution is saturated, with respect to all mineral phases.

It is the aim of this report to identify the yield in reaction products and the nature of the calcium phosphates formed by reacting phosphoric ore with phosphoric acid and extracting the available phosphorus in aqueous solution.